1. Technical Field of the Invention
The present invention relates to an alternator (on-vehicle AC generator) which Is mounted on an engine installed in vehicles such as automobiles and trucks, and in particular, to an alternator equipped with a brush and slip ring structure to deliver the power.
2. Related Art
Most engine-driven vehicles are generally equipped with alternators (on-vehicle AC generators), which generate power responsively to the rotation of the engines. Generally known conventional alternators are generally equipped with a rotary shaft and a pair of stator and rotor covered by a housing. The rotary shaft fixedly passes the rotator, so that both the rotary shaft and the rotor can rotate together around a central longitudinal axis passing the sectional center of the rotary shaft. The rotator is located inside the stator in the radial direction of the stator and is opposed to the stator with a predetermined clearance therebetween. Cooling fans are secured on the rotator and generate cooling wind by the rotation of the rotator. Thus rotating the rotary shaft allows the cooling fan to generate the cooling wind, and the generated cooling wind Is discharged through a discharge window formed through the housing. The slip rings are fixedly secured on the rotary shaft for rotation together with the rotation of the rotary shaft and the brushes are disposed within the housing to slide on the slip rings. The brushes are accommodated in a brush holder. A rear cover is also provided to cover electric components including the brushes.
In this kind of conventional on-vehicle alternators, there are provided various improvements in relation to the brushes and/or the slip rings. For example, Japanese Utility Model Laid-open Publication 58-51663 provides a brush holder with an end face formed to have labyrinth-like grooves with plural bent portions, thus securing air-ventilating passages and preventing brushes from being watered. This measure is taken to suppress abrasion of the brushes due to an Increase in the temperature thereof. Further, Japanese Patent Publication (examined) No. 7-32571 provides an arrangement of shielding members between a brush holder and a rear cover, where the shielding members have cooling-air passages. Japanese Patent Laid-open Publication No. 5-199701 shows shielding members with cooling-air passages having a labyrinth-like form including plural bent portions. In addition, Japanese Patent Laid-open Publication No. 2000-139058 teaches a mechanism In which a slip ring cover is partly inserted into a recess of a brush holder so as to form a labyrinth structure.
However, the above various structures disclosed by the conventional references have faced a concern that foreign matters (e.g., mud, dust, water) cannot be prevented well from coming into an inner space in which the brushes are positioned. This concern is especially true of off-road vehicles, because such off-road vehicles are frequently made to run on rough roads. If such foreign matters are allowed to come into the inner space, there arises a problem that environmental resistance of the brushes is weakened. One counter measure for this problem could be to Install a foreign-matter-proof casing (such as water-proof casing) in a vehicle in such a manner that the casing covers an alternator without openings. However, this installation has a further difficulty, because passages for charging and discharging cooling air should be left, and thus difficult to prevent foreign matters (such as water) from coming in. Installing the above casing also increases costs for manufacturing alternators.
Hence, there has been a demand for alternators which can be produced with lower manufacturing costs, a relatively simple structure to prevent foreign matters from coming in so that the environmental resistance performance of the alternator is enhanced.
On the other hand, when, in the inner space containing the brushes, powder dust is caused by slide operations between the brushes and the slip rings, it is desired that the powder dust is smoothly discharged outside.